Fan based turbine engines, such as those utilized on commercial aircraft, include a fan/compressor connected to turbine sections of the turbine engine via a low shaft. The turbine sections cause the low shaft to rotate, which in turn causes the fan/compressor to rotate and draws air into the fan based turbine engine. In order to control the speed of the fan/compressor, and thereby control airflow through the turbine engine, a magnetic fan shaft speed sensor is utilized in conjunction with a controller. The magnetic low shaft speed sensor monitors the rotational speed of the low shaft and the controller makes corresponding adjustments to control the low shaft speed based on the monitored speed.
In a typical arrangement, the low shaft includes multiple sensor teeth that extend radially out from the low shaft. The teeth are arranged circumferentially around the main shaft body. The teeth and the shaft are an integral monolithic component. A magnetic sensor is located adjacent to the shaft, aligned with the sensor teeth, and detects each sensor tooth as the sensor tooth rotates through the magnetic field generated by the magnetic sensor. The sensor is preloaded with the number of sensor teeth on the shaft and determines that one full rotation of the shaft has occurred when the preloaded number of sensor teeth has been detected. Using this arrangement, the speed of the shaft can be determined by the magnetic shaft speed sensor according to conventional techniques.
Due to the inherent sensing capabilities of the magnetic shaft speed sensor, the shaft in this arrangement is required to be constructed of a ferrous material, such as steel, or the magnetic sensor will be unable to detect the teeth. A steel shaft is inherently heavier that alternate, non-ferrous, shaft materials, such as titanium alloy.